The Caballos Novaculite Revisited: Part II: Chert and Shale Members and Synthesis

Abstract

ABSTRACT The chert and shale members consist largely of rhythmically intercalated beds of green or gray chert 1 to 15 cm thick separated by shale partings 1 to 5 mm thick. Chert beds are composed of equant grains of microcrystalline quartz, minor amounts of illite and Radiolaria, and trace amounts of pyrite, carbonate and other minerals and organic matter. Chert beds formed by the diagenetic alteration of sediment composed chiefly of biogenic silica from Radiolaria. Fractures and crackle breccias developed in gray chert following lithification; green siliceous sediment, whose lithification was impeded by clay, filled these pre-orogenic fractures. Beds of red shale, chert pebble and cobble conglomerate, sandstone, limestone, dolomite, and lumpy manganiferous and jasperitic chert make up only 3% of the chert and shale members, but are of controversial origin and environmental significance. The chert conglomerate beds, for example, are interpreted as tidal-channel deposits by RLF and as mass-flow deposits by EFM. Jasper beds are bizarre: they are lumpy uneven beds 0.2 to 2 m thick composed of cherry-red chert with local geopetal cavities, contorted laminae. manganiferous zones, cauliflowerlike quartz-filled cavities, zebraic chalcedony, lutecite, quartzine, pseudocubic quartz crystals, and filamentous structures resembling algae. These beds are interpreted by RLF as the product of diagenetic alteration of sabkha evaporite nodules and siliceous ooze during and following soil development and subaerial exposure; and by EFM as the product of diagenetic alteration of evaporite beds deposited in deep water and sandwiched between radiolarian ooze. Synthesis of evidence on the origin of both the novaculite and chert and shale members leads to contrasting interpretations of water depth during deposition. Evidence presented by RLF that favors shallow water include: mounds of possible algal origin, birdseye and Stromatactis-like structures, geopetal fabric, karst features, founder breccias, intraformational unconformities, tidal-channel conglomerate, biosparite beds, petrified logs, and jasper beds that contain evaporite pseudomorphs, paleosoil features, and fresh water algae. Evidence against deep-water deposition includes lateral changes in thickness of novaculite members; absence in novaculite of clay, Radiolaria, cosmic spherules, P and Mn; and narrowness of the Marathon trough during Caballos deposition. Evidence presented by EFM that favors deep water include: similarity of Caballos chert, red shale, and manganese concretions with deep-sea siliceous ooze, red clay, and manganese nodules; spumellarian Radioluria similar to living bathyal to abyssal forms; Helminthoida-like trace fossil; presence of slump structures and mass-flow deposits: Caballos is under and overlaid by deep-water formations; vertical sequences in Caballos of novaculite-radiolarian chert-red shale resembles the vertical sequence of oceanic deposits off spreading ridges (calcareous ooze--siliceous ooze-red clay); some plate tectonic models show the presence of oceanic crust in the greater Ouachita trough during mid-Paleozoic time. Evidence against shallow-water deposition includes absence of shallow-water mari e fossils except in one 3-m thick lithosome, absence of shallow-water trace fossils, and absence of current-formed stratification.